Patent Document 1 describes a kind of light interference system. The light interference system described in Patent Document 1 emits a measurement light to a measurement target object, and then, measures interference peaks of reflected lights from a front surface and a rear surface of the measurement target object. Further, the light interference system measures a temperature of the measurement target object based on an optical path length between peaks obtained by Fourier transform of the measured interference peaks. Herein, the light interference system described in Patent Document 1 includes a coupler. By using this coupler, a measurement light (incident spectrum) introduced through a single input terminal is split through N optical transmission paths at an output side to be emitted from N output terminals toward N measurement points. Furthermore, reflected lights (reflection spectrum) from the N measurement points are multiplexed to be detected from the single input terminal. Therefore, it is possible to easily measure temperatures of the multiple measurement points.
Patent Document 1: Japanese Patent Laid-open Publication No. 2012-063149
The coupler provided in the light interference system described in Patent Document 1 is formed typically by fusing multiple optical fibers. By way of example, this coupler may be formed by fusing N optical fibers having input terminals and output terminals, and by performing an antireflection process on the input terminals of (N−1) optical fibers or providing antireflection-processed light terminators at the input terminals of the (N−1) optical fibers.
However, in the coupler formed as described above, an intensity of the multiplexed reflection spectrum is reduced to 1/N, and, thus, measurement accuracy may be deteriorated. Accordingly, a light interference system capable of suppressing loss of reflection spectrum and a substrate processing apparatus are demanded.